Everyone needs to read : "Shots fired in Anger", by John George.
http://www.amazon.com/Shots-fired-an.../dp/093599842X
Mr. George was a combat infantryman in the Pacific. He was a competitive shooter before the war, and had very interesting observations on US and Japanese weapons.
The bottom line from a Dogface’s viewpoint, having a Garand made a big difference. The Japanese service rifle (vintage term was “Japs”) was a bolt action.
The Garand was not the only weapon out there. The BAR was beloved; the Nambu light was feared. Everyone wanted more fire power.
In the big picture, it is hard to say that any one weapon system was the toppling point. I think 80% of the combat deaths in WWII were due to artillery, so it would be hard to say that any small arm was the deciding factor.
But if I were in that hole, preparing to leave that hole, or waiting on the next Banzai charge, I sure as heck would prefer a Garand to a bolt action. Any day.